Photovoltaic devices typically have one or more layers of photoactive material sandwiched between two electrodes. Typically, one of the electrodes is transparent. The layers of photoactive material typically exhibit complementary charge transfer properties (e.g., one is an electron accepting/transporting material and the other is a hole-accepting/transporting material). At least one of the two materials is a light-absorbing material. In an organic solar cell, radiation absorbed by the photoactive layers creates an exciton (an electron-hole pair). Depending on the material, the electron-hole pair can only travel a short distance (on the order of several nm) before spontaneous recombination occurs. If, however, the electron-hole pair can reach a junction with differential electron affinity prior to recombination, then charge splitting can occur, upon which the split charges (holes and electrons) can move through the two different materials such that electrons are collected at one electrode and holes are collected at the other. The relatively short exciton diffusion length of many materials defines the scale at which a nanostructured charge-splitting network should be constructed.
Recently, organic materials, such as gels, conjugated polymers, molecules, and oligomers, have been used as photoactive materials. Layers of these materials can be easily deposited using standard polymer processing techniques.
Both organic and inorganic materials can be used to absorb light in the solar active layer, either singly, or in combination. To increase the efficiency of photovoltaic devices made with such materials, it is desirable to increase the absorbance in the solar cell active layer. To do so, a sufficiently large bulk volume must be positioned within the solar cell that is capable of absorbing most of the incoming light. However, since the absorbance coefficients of many organic and/or inorganic materials is not high, depositing a significant volume of material into a charge splitting network can significantly increase the distance through which charges must migrate, resulting in greater spontaneous recombination events prior to charge splitting, and lowering the charge collection efficiency of the solar cell. Although the exciton diffusion length is a property of a particular material, and can be altered as that material structure is modified, the potential range of tunability of the exciton diffusion length is nevertheless limited.
With or without nanostructuring, and even with chemically tuned materials, the performance of prior art PV cells is often sub-optimal for additional reasons. For example, many photoactive materials absorb light only over a fairly narrow band of wavelengths. It would be desirable for the active layer to absorb light over a broader range of wavelengths than is currently available in a single material. One way to do this is to use layers of different absorbing materials with overlapping absorption bands in the active layer. Although this can broaden the effective absorption band, aggregated layers of materials can still incur the problem of a larger than desired bulk volume in the active layer, leading again to undesired exciton recombination. To overcome this each absorbing layer may be relatively thin, i.e., less than or equal to the corresponding exciton diffusion distance. Although making such thin absorbing layers can enhance exciton transfer it can also reduce light absorption and exciton generation.
Thus, there is a need in the art, for a photovoltaic apparatus that is both highly efficient at absorbing light and highly efficient at transferring excitons to a charge-splitting interface prior to charge recombination. There is also a need in the art for a method for making such an apparatus.